1. Field of the Invention
The present invention relates to lithium secondary batteries and a process for preparing a negative-electrode active material for use therein. More particularly, the present invention relates to a lithium secondary battery having a negative electrode formed of graphite which is capable of intercalation and deintercalation of lithium.
2. Related Art
As down-sizing, energy saving and cordless function have been increasingly required for electronic devices, secondary batteries utilizing alkali metals such as lithium have been attracting more attentions as batteries for use in the electronic devices.
Where an alkali metal such as lithium is employed alone for the negative electrode of a secondary battery, dendrite (branching tree-like crystal) is generated on a dissolution-deposition surface of the metal during repeated charge-discharge cycles, i.e., during metal dissolution-deposition process. The dendrite further grows to penetrate through the separator of the secondary battery during the repeated charge-discharge cycles, thereby coming into contact with the positive electrode. This will result in a short circuit within the secondary battery.
It has been found that, if an alkali metal alloy is employed for the negative electrode of a secondary battery, the growth of dendrite can be suppressed in comparison with the case where the alkali metal is employed alone, thereby improving the charge-discharge cycle characteristics. However, the use of an alkali metal alloy cannot completely prevent the generation of dendrite and there still exists a possibility of the short circuit within the secondary battery.
In recent years, carbon materials and organic materials such as conductive polymers have been developed to be used for a negative electrode, taking advantage of a dope-undope process of alkali metal ions rather than a dissolution-deposition process or dissolution-deposition intrabody diffusion process of an alkali metal or an alloy thereof. The use of these materials makes it possible to avoid the generation of dendrite in principle, remarkably alleviating the problem of short circuits within secondary batteries.
In general, carbon is chemically stable and can be doped with either an electron donor or electron acceptor. Therefore, it is a promising material for an electrode of a battery.
Where a carbon material is used for an active material for a negative electrode, lithium ions can be intercalated between carbon layers in an intercalation ratio of one lithium atom to six carbon atoms, i.e., C.sub.6 Li at maximum. In this case, the theoretical discharge capacity per unit weight of carbon is 372 mAh/g. Carbon materials have large variations in their structure. The size and arrangement of hexagonal nets each formed with six carbon atoms varies depending on the starting material and production process.
Carbon materials used as a negative-electrode active material are disclosed, for example, in Japanese Unexamined Patent Publications No. 62-90863 (1987), No. 62-122066 (1987), No. 63-213267 (1988), No. 1-204361 (1989), No. 2-82466 (1990), No. 3-252053 (1991), No. 3-285273 (1991) and No. 3-289068 (1991). None of these carbon materials achieves the theoretical capacity described above and, hence, exhibits a satisfactory charge-discharge capacity when used as a material for the fabrication of a battery.
R. Fong, U. Sacken and J. R. Dahn reported in J. Electrochem. Soc., Vol. 137, pp. 2009 (1990) that the use of graphite as a negative-electrode active material realized a discharge capacity close to the theoretical capacity. However, graphite is not necessarily satisfactory in a practical use because the discharge capacity was measured under application of a small current in accordance with this literature.
Japanese Unexamined Patent Publications No. 4-112455 (1992), No. 4-115457 (1992), No. 4-115458 (1992), No. 4-237971 (1992) and No. 5-28996 (1993), for example, disclose secondary batteries utilizing graphite materials as negative-electrode active materials. However, none of these graphite materials attains the theoretical capacity and realizes a negative electrode having a sufficient negative-electrode capacity for a secondary battery.
Further, there have been proposed several composite materials each composed of a carbon material and other materials to be used as negative-electrode active materials. However, these materials each involve unsolved problems as described below.
Japanese Unexamined Patent Publication No. 5-21065 (1993) discloses the use of a negative-electrode comprising: lithium ions; a chalcogen compound which generates an average voltage of not greater than 2 V (Li/Li.sup.+) by intercalation and deintercalation of lithium ions; and a carbon material capable of intercalating and deintercalating lithium ions. Since the potential at which lithium ions are intercalated into and deintercalated from the chalcogen compound is higher than that at which lithium ions are absorbed into and desorbed from graphite, a battery utilizing such a negative electrode suffers from a lower battery voltage and reduced energy density.
Japanese Unexamined Patent Publication No. 4-184863 (1992) discloses a battery utilizing a negative electrode of a carbon material coated with a metal (nickel or copper). Further, Japanese Unexamined Patent Publication No. 4-259764 (1992) discloses a battery utilizing a negative electrode of a carbon composite material comprising carbon and a metal (at least one kind of metal which is not alloyed with lithium). Although the use of these negative electrodes improves charge-discharge cycle characteristics and high-rate discharge capacity after storage at a high temperature, a significant increase in the capacity of the negative electrode cannot be expected.
In Japanese Unexamined Patent Publication No. 3-216960 (1991), there is disclosed a secondary battery having a negative electrode of a porous carbon material on the surface of which a lithium layer is formed such that pores on the surface of the carbon material are not occluded, thus allowing for large-current discharge and improved cycle life and safety. Further, in Japanese Unexamined Patent Publication No. 4-39864 (1992), there is disclosed a secondary battery having a negative electrode of a carbon material impregnated with an alloy of a metal which can be alloyed with an active substance used therein. The secondary battery exhibits an increased electrode capacity and improved charge-discharge cycle characteristics and self-discharge characteristics. However, the production of the negative electrodes for these secondary batteries requires additional process steps.
In Japanese Unexamined Patent Publication No. 4-179049, there is disclosed a negative electrode of a composite material comprising a conductive polymer and a metal and/or carbon material. A battery having such a negative electrode is flexible and has a long cycle life. However, there still exists a problem of unsatisfactory charge-discharge capacity.
Japanese Unexamined Patent Publication No. 5-258773 proposes a negative electrode which is prepared by blending two or more kinds of negative electrode materials selected from Nb.sub.2 O.sub.5, MoO.sub.2, RuO.sub.2, WO.sub.2, metal lithium, LiTiS.sub.2, CuO and carbon materials that are capable of doping and undoping lithium. The negative electrode facilitates the detection of discharge termination voltage. However, the negative electrode cannot increase the capacity of a battery.
As can be understood from the foregoing, any of the aforesaid negative electrodes employing various carbon materials or graphite materials as a negative-electrode active material cannot attain the aforesaid theoretical capacity (372 mAh/g), thus failing to exhibit a satisfactory capacity. Though the use of the graphite material essentially allows the negative electrode to charge and discharge up to the theoretical capacity, the charge and discharge current is too small for a practical application. With the negative electrode comprising a chalcogen compound which generates an average voltage of not greater than 2 V (Li/Li.sup.+) by intercalation and deintercalation of lithium ions, and a carbon material which is capable of intercalation and deintercalation of lithium ions, the potential at which lithium ions are intercalated into and deintercalated from the chalcogen compound is relatively high, thereby reducing the battery voltage and energy density. Further, the composite negative electrode comprising a carbon material and a metal which is not alloyed with lithium cannot improve the negative electrode capacity. Still further, the composite negative electrode comprising a carbon material coated with lithium or a metal which can be alloyed with lithium requires additional process steps for production thereof. Yet further, the composite negative electrode comprising a conductive polymer and a metal and/or a carbon material has a reduced charge discharge capacity.
The present inventors have found that the capacity of a secondary battery can be significantly improved by employing copper oxide as a secondary active material for the negative electrode of the secondary battery. Copper oxide is a material conventionally proposed to be used as a positive-electrode active material mainly for use in a primary battery. When used as the secondary active material for the negative electrode, copper oxide behaves with a nature of a positive-electrode material for a primary battery, thereby reducing an initial charge-discharge efficiency. Therefore, a positive-electrode active material serving as a lithium supply source should be used in an excessive amount for the positive electrode when the secondary battery is assembled. This may hinder the improvement of battery capacity. Thus, it has been desired to improve the characteristics of the initial charge-discharge efficiency attributable to copper oxide in the copper oxide composite negative electrode.